The present invention is directed to radio receivers of the type employed for digital-signal reception. It particularly concerns the manner in which such receivers maintain the proper frequency relationship between the received signal's carrier and the local oscillator by reference to which the symbol content of the received signal is extracted.
In digital communication systems such as those of the type proposed for North American cellular-telephone traffic, information is transmitted as symbols encoded in the phase of the transmitted signal with respect to its carrier. For proper extraction of the symbols, therefore, the receiver's local reference must be quite close in frequency to the carrier signal; a frequency difference causes the apparent phase relationship to "rotate" undesirably.
Certain transmission protocols have the tendency to reduce this effect. In differential quadrature phase-shift keying ("DQPSK"), the information is contained, not in the absolute phase, but rather in the difference between the phase of a given sample and the phase of the previous sample. In an otherwise ideal transmission channel, therefore, the frequency difference does not present a significant problem so long as the reciprocal of the symbol period is large with respect to the frequency error. In actual practice, however, the channel is not ideal: it is subject to multipath fading and dispersion, so some type of process, such as adaptive equalization, that involves adaptive channel characterization is needed in order to extract symbols accurately from the time-varying channel. Such processes tend not to be very tolerant of significant uncorrected frequency offsets; their effect is to make the channel vary rapidly, and there are limits to the rate at which the adaptive processes can adapt. Even for DQPSK systems, therefore, accurate frequency tracking is necessary in practice.
The conventional approach to frequency tracking is to employ phase-locked-loop systems. That is, the phase of the received signal, or a frequency-translated version of it, is compared with the local phase reference, and the average phase difference over time is used to adjust the frequency of that reference. Unfortunately, phase-locked-loop systems tend to require a fair amount of time to achieve phase lock, and the result in a cellular-telephone system can be an objectionable amount of dead time when the user's equipment is "handed off from one cell, where one frequency prevails, to another cell, where the base-station equipment does not in general communicate with the user at the same frequency. This is true both for conventional, analog phase-locked-loop systems and for digital equivalents.
Certain other digital techniques perform the local-reference adjustment digitally, relying on data extracted from the carrier in order to estimate the offset. Such techniques are not applicable in a time-dispersive-channel environment, however, because the process for extracting the data depends on adaptive processes, which, as was explained above, are themselves relatively intolerant to significant frequency offsets: such techniques for adjusting the local reference frequency can function only after the local reference frequency has already been adjusted.
One digital approach, described in U.S. patent application Ser. No. 804,424, filed on Dec. 9, 1991, by Scott et al. for Timing and Automatic Frequency Control of Digital Receiver Using the Cyclic Properties of a Non-Linear Operation, employs the average phase relationship between successive received-signal samples to determine not only the frequency offset but also the sampler phase error. In order to attenuate data-dependent effects sufficiently that the frequency- and phase-dependent effects dominate, however, it is necessary to average the phase relationships over a large number of symbols, and this can result in a significant delay in achieving an accurate enough offset estimate.